1. Field of the Invention
The present invention relates to a semiconductor device having a T-shaped gate electrode and a manufacturing method thereof and, more particularly, to a semiconductor device that allows moisture resistance to be improved without causing an increased parasitic capacitance, and a manufacturing method thereof.
2. Background Art
A T-shaped gate electrode is formed by shaping a gate metal having two layers of, for example, WSi and Au, so that its section is T-shaped. The T-shaped section makes it possible to reduce the resistance of the gate electrode.
A manufacturing method of a conventional semiconductor device having the T-shaped gate electrode will be described.
First, a spacer film is deposited on a semiconductor substrate, and a groove is formed in the spacer film by a selective dry etching process. Then, a gate metal is deposited on the entire surface by a sputtering process.
Next, the region corresponding to the overhanging head portion of the gate electrode is covered by a photoresist, and the gate metal is dry-etched using the photoresist. Then, removing the photoresist and the spacer film creates the T-shaped gate electrode. Thereafter, an interlayer dielectric film is formed by a plasma chemical vapor deposition (CVD) process to cover the entire T-shaped gate electrode.
It is, however, impossible to evenly form the interlayer dielectric film on the leg portion and around the region underneath the bottom surface of the overhanging head portion of the gate electrode, since the gate electrode is T-shaped. For this reason, moisture enters through the leg portion and the bottom surface of the overhanging head portion of the gate electrode, and the WSi layer of the gate metal is eventually oxidized and corroded, leading to deteriorated characteristics of a field-effect transistor using the T-shaped gate electrode.
To solve such a problem, there has been proposed a semiconductor device with improved moisture resistance achieved by covering side surfaces of the leg portion of the gate electrode with a moisture-proof insulating film and by covering the bottom surface of the overhanging head portion of the gate electrode by a moisture-proof metal film (refer to, for example, Japanese Unexamined Patent Application Publication No. 5-299638).
In the conventional semiconductor device, the moisture-proof insulating film extends between the overhanging head portion of the gate electrode and the substrate rather than only in the vicinity of the leg portion of the gate electrode. The moisture-proof insulating film has a high dielectric constant, so that parasitic capacitance unavoidably increases, posing a problem of deterioration in the characteristics of the field-effect transistor having the gate electrode.
In addition, the conventional semiconductor device is not provided with a moisture-proof metal film on the side surfaces of the overhanging head portion of the gate electrode. Hence, intrusion by moisture through the side surfaces of the overhanging head portion oxidizes and corrodes the WSi layer of the gate metal, presenting the problem of deterioration in the characteristics of the field-effect transistor having the gate electrode.